The invention relates to a MOS gate-controlled thyristor with overcurrent protection having a MOSFET connected in series with the thyristor and a second, self-controlled MOSFET between the p-base of the thyristor and the external cathode, and wherein several unit cells of the thyristor are connected in parallel in a semiconductor wafer.
In case of an overcurrent, components with a self-limiting characteristic, e.g. the bipolar transistor, the power MOSFET and the insulated-gate bipolar transistor (IGBT), can be protected from destruction by using of external electronic equipment for shutting down. Electronic equipment of this type for shutting down is very expensive and requires a great deal of space. In addition, it frequently cannot be used because the saturation current for the components is too high in many cases. These disadvantages can be avoided with an integrated, quickly reacting overcurrent protection. Power MOSFETs with integrated overcurrent protection are suggested by R. Reinmuth and L. Lorenz: Intelligent Low Side Switch Provides Full Protection in High Current Applications. PCIM, January 1997, pp 41-49. An IGBT with integrated protective function is disclosed in the publication by R. Redl et al: Smart Driver IC Protects High-Speed IGBTs and MOSFET against Short Circuits. PCIM, February 1997, pp 28-39.
Protective measures are very important in case of a short-circuit because thyristors normally are used in the high-capacity range. In case of an overcurrent, thyristors are normally shut down by external, passive fuses. These and other methods are involved and relatively expensive because of the external wiring and also with respect to the space requirement.
MOS gate-controlled thyristors without overcurrent protection are known, for example, from German Patent 44 02 877 C2 and German Patent 196 27 122 A1, wherein the controlling MOSFETs are integrated into the thyristor structure with semiconductor technology. In addition to a MOSFET connected in series with the thyristor and henceforth called a series MOSFET, each unit cell of the thyristor contains a shutdown MOSFET, which is arranged between the p-base of the thyristor and the cathode and is activated automatically, as soon as the series MOSFET is shut down by its gate. As a result of the opened bypass to the cathode, the charge carriers are pulled from the thyristor structure, so that the component begins to block quickly. This shutdown structure corresponds to a known GTO cascode circuit.
In addition, some of the unit cells of the semiconductor structure are provided with a switch-on MOSFET, which is arranged between the n-emitter and the n-base of the thyristor and is provided with a gate that is connected to the gate for the series MOSFET. During the activation of the switch-on MOSFET and the series MOSFET, the anode-side pnp-transistor structure is driven high, so that the thyristor is fired and the component is switched on.
An MOS gate-controlled thyristor with overcurrent protection is also disclosed in the JP 09107091 A, in particular in FIG. 7 therein. The semiconductor structure (76) shown therein contains a thyristor with a series-connected first MOSFET (72) and a switch-on MOSFET (73), as well as a self-controlling overcurrent protection, as disclosed in the English-language abstract in xe2x80x9cPatent Abstracts of Japanxe2x80x9d.
The voltage on a detector (14) is used to activate the overcurrent protection. Together with a switch (13), this detector forms a parallel branch to the thyristor (76) with the series MOSFET (72). The current through the parallel branch determines the detector voltage and is used as measure for the thyristor current. The detector signal is supplied as gate-source voltage to a switch 15, preferably designed as MOSFET, which is arranged between the gate (79) of the series MOSFET (72) and the cathode (K) of the thyristor. If the detector voltage exceeds the threshold voltage of switch 15, this switch must change to the ON state and connect the gate of the series MOSFET (72) with the cathode (K) or the source of the series MOSFET (72). As a result, the voltage V2 at the gate (79) of the series MOSFET is lowered, that is to say corresponding to the voltage separation by the ON resistance of switch 15 and the gate series resistance 71. If the voltage V2 falls below the threshold voltage, then the component is shut down. In the inhibiting case, the switch 13 as well as the thyristor have to absorb all the voltage. The switch 13 therefore requires considerable expenditure.
That is obviously the reason why the overcurrent protection circuit was realized as external wiring. An integration with the aid of an IGBT for switch 13, for example, would require a large semiconductor surface, so that the saturation current that is reduced as a result of the gate voltage is still sufficient to generate the required detector voltage for switching on the switch 15.
In addition, the effectiveness of the protective circuit is in doubt, at least for a wide range of the overcurrent since the detector 14 and the MOSFET switch 15 mutually obstruct each other.
Given a normal current, the protective circuit must meet the requirement that the voltage at the detector is considerably lower than the threshold voltage of switch 15 since the current below the threshold value of switch 15 otherwise would lead to an undesirable leakage current for the external gate (79). In case of an overcurrent, on the other hand, the current in the detector branch must drop drastically and in time, so that the detector voltage clearly exceeds the threshold voltage of switch 15. Only in that case is the ON resistance of switch 15 low enough, so that the series MOSFET 72 and the complete component can be shut down or at least the current can be reduced sufficiently.
With the component according to FIG. 7 of the JP 09107091, this is not possible because the thyristor 76 does not shut down even after the MOSFET 7 is shut down, but initially continues to carry the complete overcurrent as a result of the avalanche breakdown. The gate-cathode voltage of switch 13 decreases with increasing detector voltage since it is smaller by the detector voltage than the gate-source voltage of the series MOSFET 72. As a rule, the latter is specified to be constant. As a result of the decrease in the gate voltage at switch 13, the current in the thyristor increases since the decrease in the gate voltage can be compensated only by a strong increase in the anode-cathode voltage at switch 13 . The latter (U at 13), however, is also smaller by the detector voltage than the anode-cathode voltage and thus increases at a slower rate when the current rises than the anode-cathode voltage. The current in the detector branch therefore can increase only with extremely high overcurrents to the required degree and can trigger a shutdown. The circuit therefore does not react or not quickly enough for a wide range of the overcurrent.
It is the object of the invention to create an overcurrent and excess temperature protection for MOS-controlled thyristors, which reacts quickly and is easy to realize.
The above object generally is achieved according to the invention by a MOS gate-controlled thyristor with overcurrent protection, wherein several unit cells of the thyristor are arranged in a semiconductor wafer and connected in parallel, and comprising:
an n-emitter, a p-base, an n-base and a p-emitter arranged sequentially between a cathode connection and an anode connection;
a MOSFET of the enhancement type connected in series with the thyristor and having a gate electrode that is connected to an external gate connection;
at least some of the unit cells of the thyristor containing a switch-on MOSFET that connects the n-base with the n-emitter when the thyristor is switched on;
a self-controlled overcurrent protection arrangement that detects the current flowing through the thyristor and, depending on this current, controls the gate voltage at the gate electrode of the series MOSFET; and wherein:
the self-controlled overcurrent protection arrangement is in the form of an additional MOSFET of the enhancement type integrated into the semiconductor wafer;
the drain source voltage at the series MOSFET serves as an indicator for the current and controls the additional MOSFET, in that
the additional MOSFET has its source conductively connected to the source of the series MOSFET, its drain conductively connected with the gate electrode of the series MOSFET and its gate conductively connected with the drain of the series MOSFET;
a series resistance is connected between the gate electrode of the series MOSFET and the gate connection;
when the additional MOSFET becomes conductive, the ON resistance of this additional MOSFET together with the series resistance forms a voltage divider that lowers the gate voltage at the gate electrode of the series MOSFET.
The essence of the invention is that an MOS thyristor, controlled by a series MOSFET, comprises between gate and source of the series MOSFETs an additional MOSFET, which in turn is controlled by the drain source voltage of the series MOSFETs and shuts down the series MOSFET if a specific voltage is exceeded. As a result, the component is provided with an I-U characteristic curve where the current with increasing voltage not only moves toward a saturation value, but clearly decreases starting with an adjustable voltage.
The advantage of this current limitation is that no external protective circuit is necessary. Another advantage is that even when the normal operating temperature of 20-80xc2x0 C. is exceeded, the current intensity immediately drops drastically and the energy dissipated throughout the component drops to an uncritical value.
The invention can be used with all MOS-controlled thyristors in a cascode circuit and with ESTs. These also include components on the basis of SiC or III-V compounds, such as GaAs.